The biological springs that act in parallel and in series with skeletal muscle contractile elements can significantly influence the force, power and displacement of muscle fibers during locomotion. All movements involve dynamic interaction between passive elastic structures and muscle contractile elements, but our understanding of the consequences of this interaction for normal gait is very limited. Our long-term goal is to define the mechanical influence of elastic elements on muscle force production and gait. The work proposed here focuses on how passive elastic structures within muscle influence fascicle length changes during lengthening contractions. Recent work indicates that the shape changes that occur in muscle and tendinous aponeuroses during contraction have a significant influence on the speed of contraction via a gearing mechanism associated with muscle fascicle rotation (i.e. changes in pennation angle). This effect is variable, and may be a critical yet previously unappreciated mechanism for modulating muscle speed and force in healthy pennate muscles. The project combines a unique animal model system with novel imaging modalities to make direct measurements of muscle force, length change, and dynamic shape changes both in vivo and in isolated muscles. The specific aims of the project are: 1) to test the hypothesis that muscle fiber length changes during eccentric contractions are accommodated by aponeurosis stretch and muscle fascicle rotation to significantly reduce muscle fascicle lengthening; 2) to demonstrate the role of dynamic muscle shape changes during locomotion; 3) to test the hypothesis that the protective effects of muscle shape changes are compromised in muscles with altered extracellular matrix properties. Changes in the elastic properties of connective tissue elements within muscle are associated with several neuromuscular disorders, aging, and secondary to stroke and spinal cord injury. A fundamental understanding of the influence of these elements on muscle mechanical function will inform the design of rehabilitative strategies to improve muscle-tendon function. An improved understanding of how parallel elastic elements influence the mechanical behavior of healthy muscle- tendon units may also aid in the design of prosthetic devices.